549 research outputs found

    Changes in the dynamical behavior of nonlinear systems induced by noise.

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    Weak noise acting upon a nonlinear dynamical system can have far-reaching consequences. The fundamental underlying problem - that of large deviations of a nonlinear system away from a stable or metastable state, sometimes resulting in a transition to a new stationary state, in response to weak additive or multiplicative noise - has long attracted the attention of physicists. This is partly because of its wide applicability, and partly because it bears on the origins of temporal irreversibility in physical processes. During the last few years it has become apparent that, in a system far from thermal equilibrium, even small noise can also result in qualitative change in the system's properties, e.g., the transformation of an unstable equilibrium state into a stable one, and vice versa, the occurrence of multistability and multimodality, the appearance of a mean field, the excitation of noise-induced oscillations, and noise-induced transport (stochastic ratchets). A representative selection of such phenomena is discussed and analyzed, and recent progress made towards their understanding is reviewed

    The origins of life on Earth

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    Essay review of "The Search for our Cosmic Ancestry" by Chandra Wickramasingh

    On the extraction of instantaneous frequencies from ridges in time-frequency representations of signals

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    The extraction of oscillatory components and their properties from different time-frequency representations, such as windowed Fourier transform and wavelet transform, is an important topic in signal processing. The first step in this procedure is to find an appropriate ridge curve: a sequence of amplitude peak positions (ridge points), corresponding to the component of interest. This is not a trivial issue, and the optimal method for extraction is still not settled or agreed. We discuss and develop procedures that can be used for this task and compare their performance on both simulated and real data. In particular, we propose a method which, in contrast to many other approaches, is highly adaptive so that it does not need any parameter adjustment for the signal to be analysed. Being based on dynamic path optimization and fixed point iteration, the method is very fast, and its superior accuracy is also demonstrated. In addition, we investigate the advantages and drawbacks that synchrosqueezing offers in relation to curve extraction. The codes used in this work are freely available for download.Comment: 13 pages, 7 figures, plus 4 supplementary figure

    Role of transdermal potential difference during intophoretic drug delivery.

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    Potential differences have been measured during transdermal iontophoresis in order to establish the effect of voltage, as opposed to current, on cutaneous blood flow. It is known that, even in the absence of drugs, the iontophoresis current can sometimes produce increased blood flow. The role of voltage in this process is studied through single-ended measurements (between electrode and body) of the potential difference during iontophoresis with 100-/spl mu/A, 20-s current pulses through deionized water, saturated 20.4% NaCl solution, 1 % acetylcholine, and 1 % sodium nitroprusside. It is found that the voltage needed to deliver the current varied by orders of magnitudes less than the differences in the conductance of these different electrolytes, and it is concluded that, at least for the present current protocol, the voltage as such is not an important factor in increasing the blood flow

    Liquid and solid helium.

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    Flow of superfluid 3He.

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    To push some of the newly discovered superfluid 3He through a fine tube, in order to see how it behaved, might seem a particularly obvious experiment, were it not for the daunting difficulties inherent in working at temperatures near 2 mK. In fact, it is a quite remarkable achievement in terms of experimental design and technique that has enabled R. M. Mueller, E. B. Flint and E. D. Adams to report (Phys. Rev. Lett., 36, 1460; 1976) the first studies of equilibrium flow phenomena, only four years after observation of the superfluid phases was first suspected. Their experiments were carried out in the Physics Department at the University of Florida

    In the family

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    Few twentieth-century physicists contribute usefully to more than one fairly specialized area of the subject, and those that do are usually theorists. Landau and Feynman are particularly striking examples but others, too, have found that their theoretical tools developed to deal with one particular problem (say superfluid helium) can also be applied fruitfully in other apparently unrelated areas of the discipline (the structure of neutron stars, for example). The situation for experimental physicists is quite different, in that their skills tend to be less portable. Most of them spend their entire careers working within, or close to, the same narrow specialism that they entered in their twenties. There are some notable exceptions, of course, and none more notable than W.M. (Bill) Fairbank of Stanford University in whose honour Near Zero has been compiled

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    Pauli principle in a gas.

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    A new and macroscopic demonstration of the Pauli exclusion principle, not in the familiar quantum liquids but in a gas, has been proposed by J.P. Bouchard and C. Lhuillier. Writing in a recent issue of Physics Letters A (116, 99; 1986), they suggest a series of experiments to seek an intriguing new kind of convective instability in dilute spin-pOlarized gases - that is, in gases whose constituent particles have magnetic moments mostly aligned in some preferred direction
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